A glass-ceramic is a material having at least one crystalline phase thermally developed in a uniform pattern throughout at least a portion of a glass precursor. Glass-ceramics have been known for over 30 years since being described in U.S. Pat. No. 2,920,971 (Stookey). They find application in diverse areas, an area of particular interest being the fabrication of articles used in the preparation and serving of food. Such articles include cookware, bakeware, tableware and flat cooktops.
In general, production of a glass-ceramic material involves three major steps:
1. Melting a mixture of raw materials, usually containing a nucleating agent, to produce a glass.
2. Forming an article from the glass and cooling the glass below its transformation range. PA1 3. Crystallizing ("ceramming") the glass article by an appropriate thermal treatment. The thermal treatment usually involves a nucleating step at a temperature slightly above the transformation range. This is followed by heating to a somewhat higher temperature to cause crystal growth on the nuclei. PA1 0.065-0.16% Fe.sub.2 O.sub.3 PA1 0.055-0.16% NiO PA1 0.0015-0.0029% Se PA1 0-0.0014% Co.sub.3 CO.sub.4 PA1 0.045-0.065% Fe.sub.2 O.sub.3 PA1 0.015-0.035% NiO PA1 2-8 ppm Se PA1 0-0.0014% CO.sub.3 O.sub.4. PA1 0.045-0.065% Fe.sub.2 O.sub.3 PA1 0.015-0.035% NiO PA1 2-8 ppm Se
U.S. Pat. No. 4,608,348 (Beall et al.) describes a family of glass-ceramics having potassium fluorrichterite as a primary crystal phase with a secondary crystal phase of at least 10% by volume cristobalite. The glass-ceramic materials are described as being highly crystalline, exhibiting superior toughness against fracture propagation and having a modulus of rupture in excess of 20,000 psi.
The glass-ceramics disclosed in the Beall et al. patent have compositions consisting essentially of, expressed in weight percent on the oxide basis:
______________________________________ SiO.sub.2 65-69 Al.sub.2 O.sub.3 0.75-3.5 MgO 13.5-17.5 CaO 3-4.8 Li.sub.2 O 0.5-2.0 Na.sub.2 O 1.5-3.3 K.sub.2 O 4.2-6.0 BaO 0-2.5 P.sub.2 O.sub.5 0-2.5 F 3.3-5.5 ______________________________________
Optionally, up to 5 mole percent total of compatible metal oxides, such as, SrO, TiO.sub.2, B.sub.2 O.sub.3, ZrO.sub.2 and ZnO, are contemplated. As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 are suggested as fining agents, and various known colorants may be present in conventional amounts.
The crystal phase in a potassium fluorrichterite glass-ceramic is a polymeric chain silicate in which a double, or higher order, multiple chain forms. A characteristic feature is a high MgO content which leads to rapid crystallization and a low, residual glass content. However, some residual glass content is necessary to avoid shattering of the crystallized material during cooling.
In addition to high MgO content, the compositions contain relatively high contents of K.sub.2 O, Li.sub.2 O and F, coupled with relatively low Na.sub.2 O and CaO contents. These composition considerations contribute to low thermal distortion in a formed glass article during heat treatment to convert the glass to a glass-ceramic.
The potassium fluorrichterite glass-ceramics have found commercial application in the dinnerware field. U.S. Pat. No. 5,070,043 (Amundson et al.) describes a color package for a pressed cup having potassium fluorrichterite as a primary crystal phase and cristobalite as a secondary phase in accordance with the Beall et al, teaching. The cup has a beige color designed to match a laminated dinnerware line sold under the CORNERSTONE trademark. The color package consists of:
The color package is further defined with reference to a polygon encompassing the x and y color coordinates. The minimum x value is 0.3245 and the minimum y value is 0.3333.
The present invention also provides a color package for a potassium fluorrichterite glass-ceramic in accordance with the Beall et al. patent teaching. In particular, it provides a line of glass-ceramic dinnerware having an attractive, light ivory color.